Examples of communications systems that perform data transmission between transmission/reception devices include communications systems that have a single or a plurality of transmission lines for data signals (hereinafter, referred to as data signal transmission lines) between transmission/reception devices, and communications systems that have both a data signal transmission line and a transmission line for control signals (hereinafter, referred to as control signal transmission lines), as represented by USB (Universal Serial Bus) and IEEE (Institute of Electrical and Electronics Engineers) 1394. In the former communications system, the transmission of data between transmission/reception devices occurs over the data signal transmission line, and the transmission of various control signals for starting and ending data transmission, for example, occurs over the control signal transmission line. There also are cases in which data transmission occurs using both control signal transmission lines and data signal transmission lines in order to increase the efficiency of data transmission.
An example of one such communications system is disclosed in Patent Citation 1. In this communications system, data transmission is performed continuously using a data signal transmission line or using both a data signal transmission line and a control signal transmission line. Patent Citation 1 discloses a technology for transmitting an interrupt request signal, which is a control signal, from the data reception side to the data transmission side over the transmission line on which data are continuously transmitted in this fashion using a time division multiplex transmission method. With a time division multiplex transmission method, a plurality of transmission/reception devices are assigned a time that a transmission line can be used, and this allows the data signal transmission line and the control signal transmission line to alternately function as a transmission side and a reception side. Here, if an interrupt request signal is to be transmitted to the transmission side during the reception by the reception side of data from the transmission side, then the period between each data unit from the transmission side, that is, the period that has been assigned to the reception side, is used to transmit an interrupt request signal to the transmission side. Thus, the reception side can freely and immediately send an interrupt request signal to the transmission side.    Patent Citation 1: Japanese Published Patent No. 2733242 Specification